1. Field of the Invention
The present invention relates to the field of fabrication of integrated circuits, and more particularly to integration and control of IC interconnect air cavities within interconnect stacks.
2. Description of the Related Art
A semiconductor device such as an IC (integrated circuit) has electronic circuit elements such as transistors, diodes and resistors fabricated integrally on a single body of semiconductor material. Advances in semiconductor materials and processing techniques have resulted in reducing the overall size of the IC circuit elements while increasing their number on a single body. Additional miniaturization is highly desirable for improved IC performance and cost reduction.
Typically, device interconnections in Very Large Scale Integrated (VLSI) or Ultra-Large Scale Integrated (ULSI) semiconductor chips are effected by multilevel interconnect structures containing patterns of metal wiring layers. Wiring structures within a given level are separated by an intralevel dielectric forming horizontal connections between electronic circuit elements, while the individual wiring levels are separated from each other by layers of an interlevel dielectric. Conductive vias are formed in the interlevel dielectric to provide interlevel contacts between the wiring traces and form vertical connections between the electronic circuit elements, resulting in layered connections.
By means of their effects on signal propagation delays and performance (e.g., time delay, crosstalk), the materials and layout of these interconnect structures can substantially impact chip speed, and thus IC performance. Signal-propagation delays are due to RC time constants (‘R’ is the resistance of the on-chip wiring, and ‘C’ is the effective capacitance between the signal lines and the surrounding conductors in the multilevel interconnection stack). RC time constants are reduced by lowering the specific resistance of the wiring material, and by using interlevel and intralevel dielectrics (ILDs) with lower dielectric constants k.
In particular, to further reduce the size of devices on ICs, it has become necessary to use conductive materials having low resistivity and to use insulators having a low dielectric constant (dielectric constant k of less than 4.0) to also reduce the capacitive coupling between adjacent metal lines. A typical metal/dielectric combination for low RC interconnect structures is copper (Cu) with a dielectric such as silicon dioxide SiO2 (dielectric constant of about 4.0).
Methods of manufacturing interconnects having copper containing materials have been developed where copper-containing interconnect structures are typically fabricated by a “damascene” process. In a typical damascene process, metal patterns, which are inset in a layer of dielectric, are formed by the steps of etching holes (for vias) or trenches (for wiring) into the interlevel or intralevel dielectric, optionally, lining the holes or trenches with one or more adhesion or diffusion barrier layers, overfilling the holes or trenches with a metal wiring material (e.g., copper), and removing the metal overfill by a planarizing process such as chemical-mechanical polishing (CMP), leaving the metal even with the upper surface of the dielectric. And the above-mentioned processing steps are often repeated until the desired number of wiring and via levels have been fabricated.
Fabrication of interconnect structures by damascene processing can be substantially simplified by using a process variation known as “dual damascene,” in which patterned cavities for the wiring level and its underlying via level are filled in with metal in the same deposition step. Dual damascene reduces the number of metal polishing steps by a factor of two, providing substantial cost savings. Dual damascene simply includes forming a trench and an underlying via hole.
Further, in addition to using copper, the use of low k dielectric materials are in heavy demand as they reduce the capacitance between interconnects and improve the switching speed of IC's. When forming vertical and horizontal interconnects by damascene or dual damascene techniques, one or more low k dielectric materials are deposited and pattern etched to form the vertical interconnects, e.g., vias, and horizontal interconnects, e.g., lines.
In back-end-of-line (BEOL) processing, important changes have included the replacement of low-k dielectrics with ultralow-k dielectrics such as air gaps as they have the lowest k value of any material (k value of about 1.0).
Thus, to fulfill future interconnect integration requirements with respect to time delay, cross talk, power dissipation, and to overcome packaging issues, the use of air gaps as the ultimate low-k inter metal dielectric has been widely implemented. As a result, specific areas may be defined where air gaps must be introduced in the interconnect stack.
Typically, as illustrated in FIGS. 1A-1B, integration schemes for forming air cavities within an interconnect stack, after all the levels where air cavities are required were integrated, are based on a removal technique, adapted to the sacrificial material used for the integration. For example, on an interconnect stack 10 as shown in FIG. 1A, above a substrate 12, copper metal lines 14 are integrated within an Undoped Silicate Glass (USG) 16, e.g., SiO2, as a sacrificial material with a SiC (Silicon Carbide) hard mask 18, it has been proposed to use wet or gaseous HF (Hydrofluoric Fluoride) attack 20 to isotropically attack the USG 16 and introduce air cavities 22 homogeneously within the stack 10 (FIG. 1B). Generally, HF chemistry is a technique used to remove sacrificial materials from the interconnect stack. However, different chemistry treatments may also be used, depending on the composition of the sacrificial material within the stack, such as vapour, gaseous, wet treatments, supercritical CO2 as a solvent or agent, and the like.
In order to achieve the mechanical stability of the entire interconnect stack 10, during integration as well as during the packaging of the ICs, it is desirable to precisely localize the region on the surface of the integrated circuit wafers where the air cavities are required. In other words, a region 24 is specified for the introduction of air cavities (see FIG. 1B). Typically the region 24 is found only in dense areas with narrow lines, where the best and highest propagation performances are required (FIG. 1B). An example of a resulting stack after a diluted HF attack mechanism has been performed is illustrated in FIG. 1B, in association with the hard mask 18 with the large open area 24 to define the surface region of the stack 10 initially exposed to the HF attack 20.
However, with this technique, because the HF attack 20 must reach the sacrificial material USG 16 to initiate its decomposition, the cavities 22 are first introduced at the upper-metal level. When such isotropic removal techniques are used, the regions with the air cavities 22 may become much larger than initially required (FIG. 1B).
Another concern is that the treatment duration may affect the metal line integrity due to the long removal process, for example, the long HF attack 20 exposure in the case of TiN (Titanium Nitride), a Physical Vapor Deposition technique such as Vacuum Deposited Coating, or TaN Cu (Tantalum Nitride Copper seed) diffusion barriers may affect the copper interconnect reliability.
Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above.